This invention relates generally to barrel reactors used to deposit material on a semiconductor wafer by chemical vapor deposition, and more particularly to a method for tuning a purge system of such a barrel reactor.
Barrel reactors of the type to which the present invention generally relates are used for deposition of epitaxial layers on semiconductor wafers. Epitaxy is an important process in the semiconductor material industry for achieving the necessary electrical properties of the semiconductor material. For example, a lightly doped epitaxial layer grown over a heavily doped substrate permits a CMOS device to be optimized for latch up immunity as a result of low resistance of the substrate. Other advantages, such as precise control of the dopant concentration profile and freedom from oxygen are also achieved.
Reactant gas carrying the material (e.g., silicon) to be deposited on the wafers is injected into a reaction chamber vessel of the barrel reactor where the deposition of the material onto the wafers is accomplished. Multiple wafers are held in a generally vertical orientation on walls of a susceptor so that one face of the wafer is exposed for deposition of silicon. The reaction chamber vessel is typically made of quartz. A seal plate which closes the reaction chamber, but can be moved to open the chamber for inserting or removing semiconductor wafers from the chamber, is made of stainless steel. A gas ring between the seal plate and reaction chamber is also made of stainless steel.
It is important that the layer of silicon deposited on the wafers not be contaminated with metals, such as iron, nickel and molybdenum, which can deleteriously affect the minority carrier lifetime of the epitaxial layer. The presence of non-metallic foreign particles in the layer is also to be avoided. The quartz reaction chamber vessel will not be a source of metals contamination, but the stainless steel in the seal plate and gas ring does provide a source for metals contamination. Contact with certain byproducts (e.g., HCl) of the reactant gas in the presence of residual moisture can cause the stainless steel to corrode. Corrosive agents such as HCl may also be present in the chamber as a result of it use to etch back deposits of silicon from the susceptor. Corrosion products from the stainless steel may be transported into the reaction chamber and become entrained with the reactant gas and material deposited on the wafers. Moreover, SiO.sub.2 deposits formed on the stainless steel by a reaction of the reactant gas with residual oxygen in the barrel reactor must be cleaned. Otherwise, some of the SiO.sub.2 can flake off and be deposited on the wafers.
To avoid contamination from the seal plate and gas ring, the barrel reactor is constructed to divert the flow of reactant gas away from the seal plate and gas ring by baffles and by a flow of purge gas over the areas of the seal plate and gas ring exposed to the reactant gas. The seal plate and gas ring are also water cooled. The conventional purge system comprises a single line of purge gas which branches off into multiple lines before reaching the seal plate for delivering purge gas to several different locations around the seal plate. The lines are connected to holes extending through the seal plate and opening on the underside of the seal plate for bathing in purge gas those areas of the seal plate and gas ring exposed to reactant gas.
In operation of a typical barrel reactor, the purge gas system is controlled by a microprocessor to first purge the seal plate and reaction chamber with nitrogen to rid the chamber of oxygen. Following the nitrogen purge, hydrogen is fed through the purge lines for several minutes prior to beginning the deposition cycle by the injection of reactant gas into the reaction chamber. The flow of hydrogen continues throughout the deposition cycle, and thereafter to purge reactant gas from the chamber. However before the seal plate is removed from the chamber, there is another nitrogen purge to remove hydrogen from the chamber.
The existing system of baffles and purge gas to avoid metals and particulate contamination from the stainless steel seal plate and gas ring is only partially successful. In particular, the purge systems are not adequate to purge substantially all of the oxygen and water vapor from the barrel reactor. As a result of the presence of oxygen (in the form of free oxygen or in the form of water vapor), significant deposits of SiO.sub.2 are formed on the seal plate and gas ring during a deposition cycle which must be removed after only a few cycles of operation of the barrel reactor. It has been found that substantial deposits occur in locations between the gas ring and seal plate which are spaced farthest away from the purge gas line which delivers gas to purge the space between the seal plate and gas ring. The act of cleaning the SiO.sub.2 deposits from the seal plate can cause corrosion of the seal plate. Any water introduced to the seal plate during cleaning which is not removed will promote reaction of the stainless steel with HCl. The shape of the seal plate often makes it difficult to remove all of the water.
In existing purge systems, the differences in pressure between the various purge lines can cause a siphon effect, in which reactant gases are actually drawn into one or more of the lines of the purge system and discharged against the seal plate. Often the reactant gas drawn in the system is ejected into a rotation mechanism of the seal plate, causing significant corrosion. Thus, rather than protecting the seal plate and gas ring from silicon deposits and corrosion, the purge system promotes their deterioration. In addition, the present systems do not adequately purge the reactant gas from the chamber after a deposition cycle. As a result, the stainless steel may react with HCl from the reactant gas when the reaction chamber is opened and moisture ladened air comes into contact with the seal plate.